Aiming at the problem of determining the drought-limited water level of emergency drought-resistant backup reservoirs, a model for determining the drought-limited water level was established based on the principle of wide and shallow damage with the goal of minimizing the time period average water shortage rate and the number of time periods with severe water shortage. A method and operational rules for determining the drought-limited water level of emergency drought-resistant backup reservoirs under extreme drought conditions were proposed. The proposed method was applied to the Deze Reservoir, a backup water source for Kunming City, and the degree of water supply guarantee under different schemes. The results indicate that the method can realize the optimal design of phased drought-limited water level in emergency drought-resistant backup reservoirs, and effectively reduce the adverse impact of extreme drought on socio-economic water use. By setting the drought-limited water level of the Deze Reservoir, the number of months with severe water shortage for emergency water supply in Kunming City can be reduced from 229 to 34, and the average monthly water shortage rate can be reduced from 27.58% to 12.74%, thereby mitigating the occurrence of extreme water shortages. Under extreme drought conditions, the combined use of the drought-limited water level and dead storage capacity can further significantly enhance the level of water supply guarantee, reducing the number of months with severe water shortages to 21 and the average monthly water shortage rate to 2.52%.